Physicalism is a philosophical position holding
that everything which exists is no more extensive than its physical
properties; that is, that there are no kinds of things other
than physical things. The term was coined by Otto Neurath
in a series of early 20th century essays on the subject, in which
he wrote:

"According to physicalism, the language of
physics is the universal language of science and, consequently, any
knowledge can be brought back to the statements on the physical
objects."

In contemporary philosophy physicalism is
most frequently associated with philosophy
of mind, in particular the
mind/body problem, in which it holds that the mind is a
physical thing in all senses. In other words, all that has been
ascribed to "mind" is more correctly ascribed to "brain".
Physicalism is also called "materialism", but the term
"physicalism" is preferable because it has evolved with the
physical sciences to incorporate far more sophisticated notions of
physicality than matter, for example wave/particle relationships
and non-material forces produced by particles. Some philosophers
use the term "materialism" to denote descriptions based on the
motions of matter and "physicalism" for descriptions based on
matter and world geometry (see: Stoljar 2001).

Because it claims that only physical things
exist, physicalism is generally a form of monism. In contrast, subjective
idealism, as exemplified by the metaphysics proposed by
George
Berkeley, holds that there is no physical reality at all and
that everything that exists is mental or spiritual (ie it is also
monistic, but in disagreement over the fundamental nature of that
monistic reality). Neutral
monism, a philosophy advocated by Baruch
Spinoza also states that only one substance exists, but that
this substance is neither physical nor mental.

Supervenience

Supervenience
is the most important concept within physicalism. It describes the
relationship between the fundamental objects of physical reality
and those of everyday experience as well as those of a more
abstract social nature. Subtle differences in the interpretation of
the supervenience concept underscore different schools of thought
within physicalism.

It can be seen as the relationship between a
higher level and lower level of existence where the higher level is
dependent on the lower level, such that one level supervenes on
another when there can only be a change in the higher level if
there is also a change in the lower level. (e.g. a set of
properties A supervenes upon a set of properties B when there
cannot be an A difference without a B difference). The debate in
this metaphor is to what extent the levels actually exist
independently of their fundamental lowest level - the
physical.

Superveniences establish such a relationship
between the mental and the physical, so that any change in the
mental is caused by a
change in the physical. Just as a shadow is dependent upon the
position of the object causing it, so is the mental dependent upon
the physical. Physicalism thus implies (through modal
realism) that:

No two worlds could be identical in every physical respect yet
differ in some other respect.

The corresponding conclusion about the mental
would be as follows:

No two beings, or things could be identical in every physical
respect yet differ in some mental respect.

Another description of supervenience does away
with levels altogether and rather pictures reality as a matrix or
mosaic, upon which we imply different patterns (the old levels) but
emphasising that all patterns are variations of the same implicit
reality.

However, supervenience alone is not sufficient to
establish the basis of physicalism. It is possible that mental or
other non-physical states supervene upon the physical. As this
allows for the possibility that the mind is causally inefficacious
and only contingently related to the physical, supervenience
physicalism is compatible with epiphenomenalism.
However, when supervenience physicalism and token physicalism are
combined, minimal physicalism is met, as will be detailed in the
following sections.

Token and Type

Token physicalism

Token physicalism is synonymous with
Property
dualism. Token physicalism states "for every actual particular (i.e. object,
event or process) x, there is some physical particular y such that
x=y". This does not entail nor is entailed by supervenience,
although if supervenience is true, it does not necessarily rule out
token physicalism. The difference between supervenience and token
physicalism is simple; token physicalism states that for every
mental particular there is a physical particular to which it is
identical, while supervenience physicalism states that set A (e.g.
mental properties) cannot change unless set B (e.g. physical
properties) changes as well. (i.e. A supervenes on B). As the name
suggests, this is a dualistic conception of reality that does not
discount the option of physical properties also having
non-supervened mental properties. Supervenience physicalism
certainly does rule out this possibility.

Still, token physicalism presents at least two
problems. It requires that for social, moral, and psychological
particulars there must be a physical particular identical with
them. Consider the Supreme Court of the United States. The Supreme
Court exists, but according to token physicalism, there is a
physical object that is identical to the Supreme Court. However,
this physical particular does not necessarily exist in any
conventional use of the word 'physical'. Supervenience escapes this
problem as the social, moral, and psychological particulars are
said to supervene on the physical particulars that compose them.
Another problem is that token physicalism does not capture minimal
physicalism, meaning that it does not capture the core commitment
of physicalism, i.e. that everything is physical. Simply because
every particular has a physical property does not rule out the
possibility that some particulars have non-supervenient mental
properties.

Type physicalism

Type physicalism (also known as Type
Identity Theory, Type-Type theory or just Identity Theory) is the
theory, in the philosophy
of mind, which asserts that mental
events are type-identical to the physical events in the brain
with which they are correlated. In other words,
that mental states or properties are neurological states or
properties. It is called type identity in order to distinguish it
from a similar but distinct theory called the token identity
theory.

According to Ullin Place,
one of the popularizers of the idea of type-identity in the 1950s
and '60s, the idea of type-identical mind/body physicalism
originated in the 1930s with the psychologist E. G.
Boring and took nearly a quarter of a century to finally catch
on and become accepted by the philosophical community. Boring, in a
book entitled The Physical Dimensions of Consciousness (1933) wrote
that:

The barrier to the acceptance of any such vision
of the mind, according to Place, was that philosophers and
logicians had not yet taken a substantial interest in questions of
identity and referential identification in general. The dominant
epistemology of the
logical
positivists at that time was phenomenalism, in the
guise of the theory of sense-data. Indeed Boring himself subscribed
to the phenomenalist creed, attempting to reconcile it with an
identity theory and this resulted in a reductio ad absurdum of the
identity theory, since brain states would have turned out, on this
analysis, to be identical to colors, shapes, tones and other
sensory experiences.

The revival of interest in the work of Gottlob
Frege and his ideas of sense and reference on the part of
Herbert
Feigl and J.J.C.
Smart, along with the discrediting of phenomenalism through the
influence of the later Wittgenstein
and J.L.
Austin, led to a more tolerant climate toward physicalistic and
realist ideas. Logical
behaviorism emerged as a serious contender to take the place of
the Cartesian "ghost in the machine" and, although not lasting very
long as a dominant position on the mind/body problem, its
elimination of the whole realm of internal mental events was
strongly influential in the formation and acceptance of the thesis
of type identity.

The type/token distinction is easily illustrated
by way of example. In the phrase "yellow is yellow is yellow is
yellow", there are only two types of words ("yellow" and "is") but
there are seven tokens (four of one and three of the other). The
thesis of type physicalism consists in the idea that mental event
types (e.g. pain in all individual organisms of all species at all
times) are, at least contingently, identical with specific event
types in the brain (e.g. C-fibre firings in all individual
organisms of all species and at all times).

If type physicalism is true then mental state M1
would be identical to brain state B1. This would imply that a
specific mental state of pain, for example, would perfectly
correlate to a specific brain state in all organisms at all times.
However, some qualify this by saying that some mental states are
not always reduced to only one specific brain state (see Putnam's
multiple
realizability). That is, the same mental state can be produced
from many different physical brain states. Token physicalism only
states that for every particular occurrence, there is a physical
particular with which it is identical. Therefore, while the mental
state of pain or happiness is not type-identical to any one
specific brain state, it is still physical and identical to a
particular brain state. It may be helpful to understand that we
often use different sets of vocabulary to describe an identical
thing, which arise out of different disciplines. For example, a
particular color, say, yellow, is a term that is identical to a
particular light wavelength within the visible electro-magnetic
spectrum. In this case to describe the actual color yellow and to
describe the same as a wavelength, is an example of a type-type
identity for they are the same thing.

Reductive and non-reductive

Reductionism is a philosophical
concept regarding the relationship between the parts of an object
and the whole.

Reductive physicalism

The physicalist variations discussed above (Token
Physicalism aka Property Dualism, and Type Physicalism aka Identity
Theory) are all ontologically
reductionist, as they reduce mental states and processes into
physical states and processes.

Reductive physicalism is not incompatible with
eliminativism -
the view that psychological states do not exist at all.

All of these types of reductive physicalism are
grounded in the idea that everything in the world can actually be
reduced analytically to its fundamental physical, or material,
basis. This is one reason why "physicalism" is often used
interchangeably with the word "materialism." Both terms (in these
instances) hold that all organic and inorganic processes can be
explained by reference to the laws of nature. The general success
of physics in explaining
a large range of phenomena in terms of a few of these basic natural
laws; such as gravity, electricity, composition of mass, has
assisted this belief..

Non-reductive physicalism

The earliest forms of physicalism, growing
historically out of materialism, were reductionist. But after
Donald Davidson introduced the concept of supervenience to
physicalism, non-reductionist physicalism became more
popular.

Non-reductive physicalism is the idea that while
mental states are physical they are not reducible to physical
properties. Donald Davidson proposed anomalous
monism as a non-reductive physicalism. Supervenience
physicalism (also proposed by Donald Davidson) is a non-reductive
physicalism, as mental events supervene (i.e. physical properties
are identical to mental properties) on physical events rather than
mental events reducing to physical events. For example if we accept
supervenience physicalism, the pain someone would feel if
electrocuted would supervene on the firing
of their c-fibres. If we
accept reductive physicalism, the pain would be those c-fibres
firing.

Emergentism is
a theory which came to popularity in the early twentieth century.
It is a form of non-reductive supervenience, but one where reality
is considered to supervene in a manner more akin to layers, rather
than patterns within a single layer, as per later physicalism.
These layers are said to be genuinely novel from each other (i.e.
the psychological vs. the physical), and is thus a type of dualism. Physicalism is
essentially monistic.

Nonreductive physicalism has been especially
popular among philosophers of biology and some biologists, who
argue that all biological facts are fixed by physical facts but
that biological properties and regularities supervene on so many
multiple realizations of macromolecular arrangements that the
biological is not reducible to the physical. Prominent exponents of
this view are Philip Kitcher and Elliot
Sober. Alexander
Rosenberg introduced Davidson's notion to the debate in 1978
but thereafter argued against nonreductive physicalism in ways
similar to Jaegwon Kim's (see immediately below).

A priori and a posteriori physicalism

Physicalism is then
further divided depending on whether it can be known
a priori or a posteriori that: If physicalism is true, S is the
statement that describes the entire physical nature of the world
collectively, and S* is the statement that describes the entire
nature of the world, then S entails S*.

A priori physicalism holds that the above can be
known without observation, i.e. independently from experience.
Originally, it was assumed that physicalism was a priori, until
Kripke argued in Naming
and Necessity for the existence of necessary a posteriori
truths.

A posteriori physicalism holds physicalism as a
necessary truth known a posteriori, i.e. known through empirical
observation. There are two main interpretations of a posteriori
physicalism which exist today. One is that a posteriori truth can
be reached a priori by contingent a posteriori truths. The other
holds that there are a posteriori truths that are taken from
non-contingent (i.e. necessary) truths. A problem arises when the
former is combined with "S entails S*", leading to a contradiction.
This view remains controversial within analytic philosophy.

Arguments for physicalism

Exclusion principle

One argument is the
exclusion principle, which states that if an event e causes
event e*, then there is no event e# such that e# is
non-supervenient on e and e# causes e*. This comes when one poses
this scenario; One usually considers that the desire to lift one’s
arm as a mental event, and the lifting of one's arm, a physical
event. According to the exclusion principle, there must be an event
that does not supervene on e while causing e*. This is interpreted
as meaning, mental events supervene upon the physical. However,
some philosophers accept epiphenomenalism, which
states mental events are caused by physical events, but physical
events are not caused by mental events. However, If e# does not
cause e, then there is no way to verify that e* exists. Yet, this
debate has not been settled in the philosophical community..

Argument from methodological naturalism

The argument from methodological
naturalism has two premises. First, it is rational to form
one's metaphysical beliefs based on the methods of natural science.
Secondly, the metaphysical world view is one that is led to by the
methods of natural science, which is physicalism. Thus, it is most
likely that physicalism is true. One reply to this argument is to
reject the second premise and state that one is not led to
physicalism by the natural sciences. However, this does not seem to
have much support. While there are other options when considering
the nature of the world, panpsychism in cognitive science, or
vitalism in biology, this is irrelevant. The argument merely states
that physicalism is the most likely, not that other views are
impossible.

Arguments against physicalism

Knowledge argument

Though there have been many objections to
physicalism throughout its history, many of these arguments concern
themselves with the apparent contradiction of the existence of
qualia in an entirely
physical world. The most popular argument of this kind is the
so-called knowledge argument as formulated by Frank
Jackson, titled Mary's
room.

The argument asks us to consider Mary, a young
girl who has been forced to investigate the world from a black and
white room via a black and white television monitor throughout her
life. However, she is allowed access to a large amount of books,
containing all physical knowledge within them. During her time in
the room, she eventually comes to know all of the physical facts
about the world, including all of the physical facts about color.
Now, to the physicalist, it would seem that this would entail Mary
knowing everything about the world. However, once she is let out of
her room and into the world, it becomes apparent that Mary does not
know everything about the world, such as the feeling or experience
of seeing color. If Mary did not have such knowledge, how can it be
said that everything supervenes upon the physical?

One way the physicalist may respond to this
argument is through the ability hypothesis, developed by Lawrence
Nemerow and David
Lewis. The ability hypothesis draws a distinction between
propositional knowledge, such as 'Mary knows that the sky is
typically blue during the day', and knowledge-how, such as 'Mary
knows how to climb a mountain'. It then states that all that Mary
gains from her experience is knowledge-how. This argument shows
that while Mary does gain knowledge from her experience, it is not
the propositional knowledge which would need to be obtained if the
knowledge argument were to be logically sound .

Argument from philosophical zombies

The zombie
argument is a thought
experiment that states "there is a possible world in which
there exist zombies". Zombies are organisms that appear to have
consciousness and
qualia, but in reality do
not. Also, in this case they have to be identical copies of
organisms in the actual world or another possible world. Though few
think zombies are nomologically possible, that is, possible in our
world, some philosophers do argue that they are metaphysically
possible. This poses a problem for the physicalist as the
metaphysical possibility of zombies would entail that mental states
do not supervene upon physical states, a claim that the physicalist
is committed to. It is then the burden of the physicalist to show
that zombies are not conceivable, or if they are conceivable, that
they are not metaphysically possible.

One argument against the conceivability of
zombies comes from Daniel
Dennett who argues that, "when philosophers claim that zombies
are conceivable, they invariably underestimate the task of
conception (or imagination), and end up imagining something that
violates their own definition". Dennett, in The Unimagined
Preposterousness of Zombies (1995) compares consciousness to
health.

However, the previous argument notwithstanding,
does the conceivability of zombies entail their possibility? One
response rests on the concept of the nature of qualia. If certain non-physical
properties exist which match our conception of qualia, then such
non-physical properties would be qualia, and zombies would be
conceivable and possible. However, if there are no non-physical
properties, then what we think of as qualia are the physical
properties which perform the functional tasks of what we conceive
of as qualia. In this scenario, zombies would not be conceivable.
Through this approach to the problem, physicalists can accept that
the possibility of zombies is conceivable, while simultaneously
denying that zombies are possible.

Jaegwon Kim against non-reductivism

In response to
Davidson's anomalous monism, Kim proposed
that one cannot be a physicalist and a non-reductivist. He proposes
(using the chart on the right) that M1 causes M2 (these are mental
events) and P1 causes P2 (these are physical events). P1 realises
M1 and P2 realises M2. However M1 does not causally effect P1 (i.e.
M1 is a consequent
event of P1). If P1 causes P2, and M1 is a result of P1, then M2 is
a result of P2. He says that the only alternatives to this problem
is to accept dualism
(where the mental events are independent of the physical events) or
eliminativism
(where the mental events do not exist).

Hempel's Dilemma

Hempel's Dilemma attacks how physicalism is
defined. If, for instance, one defines physicalism as the universe
is composed of everything known by physics, one can point out that
physics cannot describe how the mind functions. If physicalism is
defined as anything which may be described by physics in the
future, one is saying nothing.

One possible reply to this dilemma is that over
time we see more and more evidence from neurology that mental
functions are related to physical neural
correlates within the brain. Combined with the observation that
phlogiston gave way
to thermodynamics, vitalism gave way to cell
biology, and other examples of previously dualistic concepts
being eroded by continuous scientific progress, it can be argued
that the physical basis of the mind will be known sometime in the
future.